Add prettier (closes #1)

Install prettier globally beside wrangler and wrangler's peers, and
check it in the smoke test. prettier ships no distro package for
Ubuntu, so npm global is the standard install.

Model: deepseek-v4-pro

build.sh

@@ -61,7 +61,7 @@ nvm install --lts
 nvm alias default "lts/*"
 nvm use default
 set -ux
-npm install -g npm@latest corepack wrangler
+npm install -g npm@latest corepack wrangler prettier
 corepack enable
 # Pre-populate the corepack cache so projects that pin a package manager
 # do not download at run time. The last yarn installed becomes the default.
@@ -102,6 +102,7 @@ npm --version
 COREPACK_ENABLE_NETWORK=0 yarn --version
 COREPACK_ENABLE_NETWORK=0 pnpm --version
 wrangler --version
+prettier --version
 python3 --version
 pip --version
 virtualenv --version